2332
Ind. Eng. Chem. Res. 1992,31, 2332-2339
MATERIALS AND INTERFACES Accelerated Degradation Studies of Starch-Filled Polyethylene Films Wei Sung and Zivko L. Nikolov* Department of Food Science and Human Nutrition, Center for Crops Utilization Research, Iowa State University, Ames, Iowa 50011
The degradation of two different starch-polyethylene films containing about 5% by weight (wt %) corn starch was investigated. The major difference between the two films was the presence/absence of a prooxidant additive in the formulation. The accelerated starch biodegradation of the films was investigated by using a buffered Bacillus sp. a-amylase solution whereas oxidative degradation of the polyethylene was hastened by incubating the starch-polyethylene films in a forced-air oven at 70 “C.The progress of degradation was followed by monitoring physical and chemical changes of the samples by using tensile strength and elongation testings, high-temperature gel permeation chromatography, infrared spectroscopy, and scanning electron microscopy. The films without a prooxidant additive did not change significantly during the heat treatment. The films containig a prooxidant lost their physical properties after 20 days of heat treatment. The amount of starch hydrolyzed by a-amylase was directly related to physical properties of the polyethylene films. The levels of starch degradation ranged between 10 and 50 wt % of initial starch, depending on the extent of the polyethylene degradation.
Introduction Polyolefm and plastics in general have attracted much attention in recent yeara, mainly due to their widespread use as packaging materials, which led to their increased visibility as litter in the environment. Polyolefm are not inherently biodegradable, and considerable efforts have been made to accelerate biodegradation of packaging materials by using natural additivea or fillers. The strategy for inducing degradability of plastics is to use additives that will biodegrade and/or accelerate the rate of oxidative degradation of the polymer. The use of starch as a filler in plastics has been of interest for the past 30 years, but only since the mid-1970s were polymer starch blends with acceptable physical properties developed. Two major approaches to starch addition to polyethylene (PE) have emerged. The finst one uses gelatinized (native or modified) starch mixed with a poly(ethy1ene-co-acrylic acid) ( E M ) to produce blends containing up to 60% starch (Otey et al., 1977,1980,1987; Swanson et al., 1988). The second approach utilizea native or modified granular starch as filler in formulated degradable masterbatch products. Ecostar International (formerly a part of St Lawrence Starch Co., Ltd.)produces a masterbatch containing a modified granular starch and unsaturated fatty acids or their derivatives (i.e., vegetable oils) as ‘autooxidants” to promote oxidative degradation of the polyolefin according to early patents by Griffin (1977a,b). Archer Daniels Midland Co. (ADM) produces a masterbatch containing native granular starch, an unsaturated polymer as ‘autooxidant”, and trace amounts of certain transition metal (Fe, Cu, Mn) salts to catalyze the oxidative degradation of the poyolefin as described in a later patent by Griffin (1991). In this paper, the mixture of the ‘autooxidant” and the transition metal additive will
* To whom correspondence should be addressed.
be referred to as a prooxidant. Starch-PE blends produced by both methods are often claimed to be degradable without making a clear distinction between biodegradation of starch and oxidative degradation of the polymer. There are relatively few scientific publications that specifically address degradation aspects of starch-polyolefin blends. Recent degradation studies of starch-PE blends (40wt % starch) prepared by Otey’s technology showed that most of the starch incorporated was degraded when the plastic filma were buried in a moist soil (Wool and Cole, 1988; Wool et al., 1990) or were inoculated with a mixed culture of starch-degradingbacteria (Gould et al. 1990). When the same types of films were tested for starch biodegradation using amylolytic enzymes, the results varied from almost no hydrolpis (Gould et al., 1990; Allenza et al., 1990) to as much as 90% starch hydrolysis (Wool and Cole, 1988). The extent of microbial or enzymatic degradation of granular starch in starch-PE blends is even more difficult to compare because of possible interference from oxidative degradation of the polymer. Griffin (19’741, following a weight loss, estimated that 80% of the initial starch was removed during a garden soil burial experiment. Iannotti et al. (1990) studied the effect of prooxidant on starch removal from starch-PE blends that contained from 3 to 9 wt 9% starch by using Fourier transform infrared (FTIR) spectroecopy. The plastic films exposed to soil, refuse, and anaerobic environment lost between 25 and 30 wt % of their original starch content after 24 weeks, and the film samples containing a prooxidant lost Significantly more starch than those containing only starch. In a similar experiment, Maddever and Campbell (1990)reported that after 2 week more than 90 wt % of the ininitial starch was removed upon incubating the films (6 wt % starch) in an anaerobic digestor, whereas only 30% of the initial starch was lost upon incubation in soil or active compost environment. Austin (1990) evaluated degradation of starch-
0SSS-6SS6/92/2631-2332$03.00/0@ 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 10,1992 2333 PE blends containing 6 wt 90starch during a 10-month incubation period with a standardized inoculum. After 7 months of incubation, neither elongation at break nor molecular weight change of PE was observed, and about 10 wt % of the initial starch was degraded, as estimated from carbon dioxide evolution. Enzymatic hydrolysis of starch was determined to be between 10 and 14 wt % of the original starch in the blends containing from 5 up to 40 wt % (Allenza et al., 1990; Evangelista et al., 1991). The lack of standardized methods applied in the assessment of degradation of starch-PE blends, complicated by the variety of formulations and testing conditions, makes the comparison of degradation data very difficult. The goal of this work was to shed more light on the importance of prooxidant and starch additives in degradation of starch-PE blends and to attempt to explain the apparent variation of starch biodegradation data.
Experimental Section Plastic Samples Preparation. Starch-PE films (ISU films) containing 5 wt % native corn starch were prepared as described earlier (Evangelista et al., 1991). A mixture of industrial-grade corn starch dried to 0.22% moisture and Dow PE-2045 linear low-density polyethylene (LLDPE) was compounded and cast at 205 "C and a screw speed of 20 rpm. The other set of samples was prepared by using ADM Polyclean masterbatch blended with LLDPE to produce a 6 wt % starch film (ADM films). The latter films were commercially prepared for us by Home Plastics (Ankeny, IA) according to ADM-recommended specifhtions. ISU films did not contain any other additives besides starch and averaged 112 pm (4 mil) thick, whereas ADM films contained a prooxidant and had an average thickness of 88 pm (3.5 mil). The actual starch content of ADM films was estimated to be 5.5 wt %, whereas that of ISU films was estimated to be 4.6 wt % (Fratzke et al., 1991). Enzyme Activity Assays. Starch biodegradation tests were performed by using a four-time crystallized Bacillus sp. a-amylase (Sigma, St. Louis, MO). Enzyme activity was determined by measuring the formation of reducing sugm by using a 2% (w/v) soluble corn starch (National Starch and Chemicals, Bridgewater, NJ) solution in 0.05 M acetate buffer (pH 6.0) as a substrate. The reducing sugar concentration was measured by using the Somogyi-Nelson method (Nelson, 1944) with maltose (Fluka, Ronkonkoma, NY) as a standard. One unit (U) of aamylase was defined as the amount of enzyme that generated reducing sugars from starch equivalent to 1 pmol/min maltose at 37 "C and pH 6.0. Enzyme stability was tegted, and it was determined that the activity did not change significantly during 72 h of incubation in 0.05 M acetate buffer (PH 6.0) at 37 OC. The ability of this enzyme to hydrolyze native (insoluble) starch was confirmed under the same experimental conditions by using a 2% (w/v) corn starch solution. Degradation Assays. The effect of oxidative degradation on film properties was studied using the oven aging test, which involves heating of plastic samples in the presence of oxygen at temperatures ranging from 70 to 90 OC (Maddever and Campbell, 1990). A temperature of 70 OC is commonly used because it is comparable to the average temperature of an active compost environment and the test, then, is directly correlated to the performance of the material in an active compost. Plastic films were cut into 2.5- X lbcm strips, placed in 20-mL glass scintillation vials with loosely screwed caps, and subjected to heat treatment in a forced-air oven at 70 OC for a maximum of 90 days. To minimize further oxidation, films were stored
in sealed glass vials at -15 OC before any further treatments or analyses were performed. The susceptibility of starch to enzymatic attack and the extent of starch hydrolysis in the starch-PE blends was determined as described previously (Evangelista et al., 1991). The films were cut into 1.2- X 1.2-cm squares, and approximately 100 mg was weighed to fO.l mg and transferred into a 250-mL beaker. The reaction mixture consisted of 25 mL of 0.05 M acetate buffer (pH 6.0), 720 U of Bacillus sp. a-amylase (29 U/mL), and 54 mM CaC12-2H20.A l-mL aliquot of the reaction mixture was analyzed for total carbohydrate (Dubois et al., 1956) every 24 h in the first two days and every 48 h thereafter. At the end of each 24- or 48-h interval the reaction mixture was discarded and replaced with a fresh one. Control samples were treated in the same manner except without a-amylase. The amount of starch removed from the films into the buffer solution was determined by the total carbohydrate assay. At the end of treatment, the f h squares were air-dried at room temperature for 48 h and stored at -15 OC for further analyses. Film Testing. Mechanical properties (tensile strength and percent elongation at break) of starch-PE films were analyzed by using an Instron Universal Testing Instrument Model 4502 (Canton, MA) equipped with a l-kN static load cell. Plastic films were cut in machine direction into 2.5- X 7.5-cm strips with smooth edges. Each film strip was marked 1.25 cm from each end and then covered with masking tapes so that 5.0 cm of the film was exposed between the masking tapes. The thickness of each strip was measured with a Craftsman precision machinists' micrometer (Chicago, IL)to f0.0025 mm (*0.1 mil). Test samples were conditioned for 48 h in a 50% relative humidity chamber. Tensile strength and elongation were measured immediately after removal of samples from the humidity chamber. The initial grip distance was 5.0 cm, and the test speed was set at 5.0 cm/min according to the ASTM D882-83 method. Tensile strength and elongation were calculated by using Series IX Automated Materials Testing System Version 4.09 software (Instron, Canton, MA). Fourier Transform Infrared (FTIR) Speotroscopy. The extent of PE oxidation was followed by measuring the levels of ester carbonyl (1735 cm-') and ketone carbonyl (1715 cm-') absorbances by FTIR spectroscopy. Levels of them carbonyl compoun& in PE films were measured with a Bruker Instruments Model IR 113V spectrometer (Billerica, MA), controlled by Bruker IFS Version 12/87 software. The thiclmw of each piece of film was measured to f0.0025 mm (fO.l mil) with a Craftsman precision machinists' micrometer (Chicago, IL). PE films free from wrinkles were mounted on standard FTIR sample plates by using removable tapes in such a way that the hole was covered. The sample chamber was purged with dry air for several minutea before the collection of each interferogram. Satisfactory interferograms were obtained from 128 scans collected at a resolution of 1.0 cm-' by using an aperture of 3 mm. A Happ-Genzel apodization function and a zero-filling factor of 2 were used in computing singlsbeam spectra. Reference single-beam spectra were generated by using an empty sample chamber. Carbonyl index, defined as the ratio of carbonyl and methylene (1465 cm-') absorbances, was used to express the measured levels of carbonyl compounds (Albertsson et al., 1987). Defining the two carbonyl indexes, A1T16/A1- and A1,%/A1-, relative to the invariant methylene (1465 cm-') absorbance compensated for the variation in the thickness of the starch-PE samples (Albertsson et al., 1987).
2334 Ind. Eng. Chem. Res., Vol. 31, No. 10,1992
High-Temperature Gel Permeation Chromatography. The change of molecular weight distribution of PE was determined by using a Waters 1 5 0 4 gel permeation chromatograph (Milford,MA) equipped with three Waters pStyragel HT Linear GPC columns with a functional molecular weight range of 500 to 8 X loe connected in aeries. An aliquot of 200 pL of sample previously dissolved in 1,2,4trichlorobenzene(TCB) was injected in duplicates. TCB was used as a mobile phase at a flow rate of 1.0 mL/min and temperature of 140 OC. Since a constant flow rate was critical, a low molecular weight marker such as decalin or n-dodecane was used to indirectly monitor the flow rate. Total processing time for each sample was approximately 55 min. A refractive index (RI) detector sensitivity setting of 128 was used for runs with both the molecular weight standard and PE samples. Numberaverage (M,) and weight-average (M,)molecular weights were calculated relative to narrow molecular weight polystyrene standards with molecular weights ranging from 2700 to 600000 and M,/M, I1.1by using Dynamic Solutions Maxima 820 software (Millipore, Milford, MA). For more details about sample preparation refer to Lee et al. (1991). Scanning Electron Microscopy (SEM). Examination of the surface microstructure of starch-PE films was performed with a JEOL JSM-35 (Tokyo,Japan) scanning electron microscope operating at 15 kV and 80 pA. The magnifications used ranged from 500X to 3300X. A Polaron SEM Coating Unit Model E-5100 (England) was used to coat the specimens with a thin layer of 60% gold (Au) and 40% palladium (Pd) under vacuum for 4 min. Statistical Analysis. To determine if heat treatment significantly increased the extent of starch removal, a simple t-test was used. Starch removal data for each heat treatment were treated as one set of samples, and their mean and standard deviation were computed. The mean of each set of samples was compared to every other mean using the paired t-test at the 95% confidence interval. Results Starch Removal from Starch-PE Films. The amounta of starch removal from ADM films during 1week of incubation with a-amylase are plotted in Figure la. Control samples, shaken in buffer solution in the absence of a-amylase, also lost certain quantities of starch, which are reported as starch leaching in Figure lb. The contribution of a-amylaae activity to starch removal from the films in Figure 1can be computed from the difference of starch loas between the enzyme-treatedsamples and their corresponding controls. The amounta of hydrolyzed starch in the € i that were not subjected to heat treatment (0-day sample) and those that were heat-treated for 8 and 12 days did not differ significantly (P< 0.05)(Figure la). The enzymatic hydrolysis of starch in the 0-day films was about 10 wt % after 7 days of incubation, and starch leaching from the control was almost 2 wt % . During the same incubation period, 8- and 12-day heat-treated films (Figure la) lost 12 and 14 wt % of the initial starch to enzymatic hydrolysis, respectively, whereas the corresponding controls released less than 5 wt % starch, as shown in Figure lb. A significant increase (P< 0.05)in starch removal by hydrolysis and leaching was observed for the films heated for 20 and 30 days. At the end of incubation period of the 30-day heat-treated films, almost 50 wt % of the initial starch was removed by hydrolysis (Figure la), and only 20 wt 9% starch was released by leaching from the control (Figure lb). ADM films that were heat-treated for 60 and
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90 days became very brittle and disintegrated after 7 days of shaking in the buffer, releasing almost 50 wt % of the initial starch (Figure lb). These films were therefore not subjected to enzymatic attack because of their disintegration and considerable starch loss. The resulta in Figure 1indicate that the length of heat treatment at 70 "C was directly related to the total release of starch from starchPE films and that a-amylase activity would be of less importance in the overall starch removal process when the films are heat-treated for more than 30 days. The rates of starch hydrolysis were the highest during the first 24 h of incubation, and with the exception of the 30-day heat-treated films, all starch granules accessible to the enzyme were removed after 48 h of incubation (Figure la). The rates of starch leaching from films that underwent heat treatment for more than 20 days were distinctly higher than the rates of starch leaching of 8- and 12-day films. ISU films, which did not contain a prooxidant additive, lost much smaller amounts of starch by either leaching or a-amylase activity as compared with ADM films (Figure 2). The incubation period of ISU films was extended to 2 weeks because of the lower levels of starch hydrolysis (Figure 2a). A t the end of the incubation period, starch hydrolysis ranged from 8 to 15 wt % of the initial starch, but the measured levels of starch removal were not significantly (P < 0.05)different (Figure 2a). In addition, the levels of starch leaching were much lower than those of ADM films. After 14 days of incubation, the amounts of starch released from heat-treated films varied between 1 and 5 wt % starch (Figure 2b), but these variations were not significantly different (P< 0.05). For ISU films, the quantities of starch lost in the presence or absence of a-amylase did not correlate with the length of heat treatment. Surface Microstructure. SEM was used to examine the effect of heat treatment and enzymatic hydrolysis on
Ind. Eng. Chem. Res., Vol. 31, No. 10,1992 2335 16
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Figure a. Scanning eleetmn micrographs of O-day heat-treated ADM films before (a) and after (b) enzyme treatment. Partly exposed (E) and well-eovend (embedded) starch granule8 (W)am indicated by nrrom. Bar,16 Irm.
surfam of starch-PE f h ( F i i 3). Partly exposed and well-eovered starch granules before the enzyme treatment are shown in Figure 3a. Empty craters (Figure 3b) presumably resulted from hydrolysis of the granules that were accessible to a-amylase activity. Covered starch granules, shown as round protrusions on the film surface in Figure 3, remained intact. The effect of heat treatment and shaking on the surface structure of the beat-treated ADM films is shown in Figurea 4 and 5. The length of heat treatment evidently did not affect surface texture because the f h that underwent heat treatment for 0,30, 60, and 90 days do not exhibit noticeable difference in surface texture (Figures 4a, 4c,5 4 and 5c). However, when these films were subjected to shaking in buffer, they developed noticeable surface rug-
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Figure 4. Effect of heat treatment and shaking on the surface texture of O-day (a and b) and 3O-day (c and d) heat-treated ADM f h . Scanning electron (SE) micrographs a and c show the surface texture of f i i s after heat treatment, whereas micrographs b and d show the surface texture of the heaetreated f i h after I days of shaking in buffer. Bar,4 pm.
gedness (Figures 4b, 4d,5b, and 5d). In a separate set of experiments, the same heat-treated samples were incubated in the same buffer without shaking. The 60- and %day (but not the 0- and 30-day) heat-treated films exhibited surface ruggedness, indicating that water somehow contributed to this process. Molecular Weight Change. The heat treatment of ADM films, which contained a prooxidant, resulted in a significant decrease of the molecular weight (MW) of PE (Figure 6a). Changes in weight-average molecular weight (M,) paralleled changes in number-average molecular weight (M.) (Figure 6a). The molecular weight decrease was not monotonous; after 8 days of heat treatment, there was a slight increase in both M, and Mn,followed then by a rapid and continuous decrease. The most drastic reduction in molecular weight, an 80% decrease, occurred between the 8th and 20th days of heat treatment. Polydispersity index (MJMJ, representing the width of MW distribution, also decreased significantly during the same period, as shown in Figure 6a. The polydispersity index of PE was initially 5.5, which reflected a rather wide MW distribution, but after 20 or more days of heat treatment, this value was reduced to about 2. For ISU films, which did not contain a prooxidant, the MW of the PE remained essentially unchanged even after 90 days of heat treatment (Figure 6b). Change of Mechanical Properties. The mhet common method for assessing degradation of plastic materials is measwing changes in their mechanical properties. ADM f h showed decreases in tensile strength and elongation (Figure 7a) that generally paralleled changes in the MW of corresponding films. ADM films lost all their initial tensile strength and elongation after 30 days of heat
2336 Ind. Eng. Chem. Res., Vol. 31, No. 10,1992 35
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treatment at 70 O C Q i i 7a). Elongation did not change during the fmst 6 days of heat treatment, whereas tensile
Figure 1. Changeg in mechanical propertias of ADM f h (a) and ISU f h s (h) during heat treatment at 70 OC. Dotted lines in (a)
represent unmensurable values. Average of four m e n s m e n t a
strength decreased to almost half the initial strength during the same period. On the other hand, ISU f i i , which did not contain a prooxidant, exhibited essentially no changes in mechanical properties (Figure 7h). Fluctuations of the values of tensile strength and elongation were within limits of experimental errors. Polyethylene Oxidation. Formation of carbonyl compounds as products of PE oxidation was easily detected hy IR spectroscopy because carbonyl compounds absorb strongly around 1700cm-' (Figure 8a). Height of carbonyl peaks increased with the length of heat treatment up to 60 days, after which it remained constant (Figure 8h). In addition, two peaks, identified as ketone carbonyl (1715 cm-') and ester carbonyl (1735 cm-') peaks, were clearly shown in Figure 8b. Plots of carbonyl index (CI) as a function of the length of heat treatment are presented in Figure 9. After a 16-day lag period, the concentration of ketone carbonyl groups in the 20-day ADM sample suddenly increased from approximately 0.1 to 1.0. The ester CI followed the increase of the ketone CI, but it was always at least an order of magnitude smaller than the ketone CI. The initial rate of ketone carbonyl formation was much higher than that of the ester carbonyl. Changes in the CI paralleled those of mechanical properties and MW of PE. Carbonyl indexes of ISU fih consistent , with the MW and mechanical properties measurements, remained very low and essentially unchanged during the heat-treatment process (Figure 9). Discussion Hydrolysis and Leaching of Starch. Properly processed starch-PE films should have all starch granules either embedded in or completely covered with PE. The number of starch granules that protrude from the film surface depends on the size of starch g~anulesrelative to
Ind. Eng. Chem. Res., Vol. 31,No.10,1992 2337
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Wavenumbers (cm-‘) Figure 8. IR spectra of heat-treated ADM films. (a) Entire IR spectrum of the 90-day heat-treated ADM film. (b) IR spectra of 0-90-day heat-treated ADM films in the methylene and carbonyl region-bordered by dotted lines in (a). Peaks at 1465,and 1735 cm-I denote methylene, carbonyl ketone, and carbonyl ester peaks, respectively.
the film thickness as well as the starch concentration in the film. Theoretically, very little water is expected to diffuse into the PE film and hydrate starch granules because of the low permeability of polar water molecules in the nonpolar PE matrix (Jasse, 1986). The results of starch leaching given in Figures 1 and 2 indicate that less than 4% and about 2% of starch were released from the @day heat-treated ISU and ADM films after shaking in buffer, respectively. These small amounts of starch removed probably were due to partially expoeed starch granulea that were not completely coated with PE and therefore were subjected to eroeion/solubilization by water. Microscopic examination of both ADM and ISU films confirmed the existence of these exposed starch granules. The addition of a-amylase to the &day heat-treated ADM and ISU films resulted in removal of about 10 wt 9% starch owing to hydrolytic activity of the enzyme (Figures 1 and 2). We hypothesize that the enzyme attacked exposed granules on the f i b surface. In addition, swollen starch granules (Swinkels, 1986) that eventually ruptured the thin PE coating were also susceptible to enzyme attack. In our previous work (Evangelista et al., 1990), we measured between 10 and 13 wt % of initial starch hydrolysis in ISU films containing from 5 to 25 wt % starch. The small extent of starch biodegradation observed in our study involving ISU films was consistent with results other studies (Austin, 1990;Allenza et al., 1990; Wool et al., 1990). The accumulated information on starch degradation from our and other studies confirmed the percolation analysis of Peanasky et al. (1991),who demonstrated that starch
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concentrations greater than 40 wt % were required to make all the starch granules in starch-PE films accessible to enzymes. Effect of Oxidative Degradation on Starch Removal. ISU films, containing no prooxidant, did not exhibit any correlation between length of heat treatment at 70 O C and the amount of starch removed by either enzymatic hydrolysis or leaching (Figure 2). These results are consistent with the insignificant changes (P< 0.06) of molecular weight (Figure 6b), mechanical properties (Figure 7b), and levels of oxidation products (Figure 9b) of ISU films. Starch removal from ADM films by either enzymatic action or leaching was related to the length of heat treatment (Figure l), which, in turn, affected physical properties of the starch-PE films. Statistical analysis of the data presented in Figure 1revealed that the first significant increase in starch hydrolysis and leaching occurred after 20 days of hear treatment at 70 OC. This observation parallels the rapid drop in MW of PE between the 8th and 20th days of heat treatment, shown in Figure 6a. After 20 days of heat treatment, the M, decreased to almost one-eighth of the initial M,, indicating the occurrence of an extensive PE degradation. Another evidence of extensive PE oxidation was the increase in CI after 20 days of heat treatment, as shown in Figure 9. The pattern of changes in both M W distribution (Figure 6) and CI (Figure 9)of PE suggests that little degradation of PE films occurred during first 12 days, and this explains why no significant difference in the extent of starch hydrolysis and leaching was observed in these samples (Figure 1). As the oxidative degradation of PE further progressed, 80 did the starch removal from ADM films. The progression of film deterioration as a function of the length of heat treatment can be visualized on SEM micrographs of the 0-, 30-, 60-,
2338 Ind. Eng. Chem. Res., Vol. 31, No. 10, 1992
and 90-day heat-treated films that were shaken without the enzyme (Figures 4 and 5 ) . The increased surface ruggedness parallels the levels of starch removal from these (Figure 1). After 60 and 90 days of heat treatment, no further measurable changes of the film properties occurred as indicated by M , and CI values. Deteriorated PE films apparently allowed water and enzyme to access previously embedded starch granules, which results in increased starch removal. The swelling of protruding starch granules probably led to the rupture of the deteriorated PE and subsequently to the removal of the starch granules from the films. This seems to be a possible explanation for the 60-day and Wday heat-treated controls, which already lost 30 wt % starch during the first day of shaking. A similar relationship between the starch removal and oxidative degradation of PE films was also observed by Iannotti et al. (1990). Oxidative Degradation of PE. The increased starch leaching and susceptibility of starch to a-amylase hydrolysis in starch-PE films have been attributed to the oxidative degradation of PE. The deterioration of starch-PE films due to the PE degradation was monitored by measuring changes in the mechanical, physical, and chemical properties of the film. As mentioned earlier, ISU films apparently were not oxidized, and no measurable changes of the film properties were detected. The stability of ISU films to oxidation is probably due to antioxidants, which are usually present in commerical polymers. Loss of tensile strength and elongation of ADM films was observed immediately after the heat treatment was initiated (Figure 7a). The tensile strength decreased sharply in the first 10 days of heat treatment, whereas the elongation showed a 6-day lag period and then also decreased rapidly, reaching a minimum after 20 days of heat treatment. The loss of mechanical properties was consistent with the decrease of the polymer MW as indicated in Figure 6a, although the MW decrease was not monotonous. After the initial drop in M W ,there was a slight increase in both M , and M , around the eighth day of heat treatment followed then by a rapid and continuous decrease (Figure 6a). A similar increase in MW during PE oxidation has been observed before, and it was attributed to polymer crosslinking (Holmstrom et al., 1978; Lee et al., 1991). Reduction in polydispersity index (PI),which reflects the width of the MW distribution, suggests that larger PE molecules were preferentially attacked throughout the degradation process. FTIR spectroscopy of ADM films revealed the formation of two major carbonyl groups, which were identified as ketone (1715 cm-') and ester (1735 cm-') carbonyls (Figure 8). The concentrations of both ketone and ester carbonyl groups in ADM films increased significantlyafter the 20th day of heating (Figures 8b and 9a). The extent of PE oxidation expressed as CI correspondsto the observed loss in mechanical properties and the MW decrease of PE. Our results indicate that very little ester carbonyl was formed during the oxidation of PE and that the ketone groups were the predominant oxidation product. This observation is in general agreement with the other published results (Benham et al. 1976;Albertason et al., 1987),although Iring and Tudos (1990) found carboxyl groups to be the predominant product of PE oxidation.
Conclusions The original purpose of investigators who developed PE-starch blends technology was to enhance degradation of PE when disposed of in biologically active environments. It was envisioned that the initial biodegradation of starch
would provide a greater surface area in the films for oxidative breakdown of the polymer, followed by microbial digestion of the degradation products. Our work as well as the most recent data from other studies suggest that about 10 wt % of initial starch was removed from commercial films, which usually contain a maximum of 10 wt 5% starch. Starch loadings of as high as 25 wt % (ISU films) did not result in increased starch biodegradation. This work showed that the presence of a prooxidant additive was essential for initiating the PE degradation,which then led to much higher levels of starch removal from ADM films. Therefore, the often claimed rapid disintegration of plastics in the environment probably was due to synergistic actions of oxidative degradation of the polymer and starch biodegradation enhanced by mechanical forces. In our opinion, for starch to act as an enhancer of the degradation of PE-starch blends without the help from prooxidant additives, loadings higher than 40 w t % starch should be employed as suggested by the percolation analysis of Peanasky et al. (1991).
Acknowledgment We thank Dr. Anthony L. Pometto I11 of Iowa State University for reviewing the manuscript and helpful discussions during this study. We also thank Dr. Alfed Fratzke for his assistance with FTIR and GPC analysis. This research was supported by the Iowa Corn Promotion Board, the Iowa State Legislature, the ISU center for Crops Utilization Research, and the Iowa Agriculture and Home Economics Experiment Station. This is Journal Paper No. 5-14823 of the Iowa Agriculture and Home Economics Experiment Station, Ames; Projects No. 0178 and 2863.
Nomenclature CI = carbonyl index MW = molecular weight M, = weight-average molecular weight M, = number-average molecular weight PE = polyethylene Registry No. PE-2045, 26221-73-8; starch, 9005-25-8; aamylase, 9000-90-2. Literature Cited Albertason, A. C.; Andersson, S. 0.;Karlason, S. The mechanism of biodegradation of polyethylene. Polym. Degrad. Stab. 1987,18, 73-87. Allenza, P.; Schollmeyer, J.; Rohrbach, R. P. Evaluating biodegradable plastics with in vitro enzyme assays. In Degradable Materials: Perspectives, Issues and Opportunities; Barenberg, s. A,, Brash, J. L., Narayan, R., Redpath, A. E., Eds.; The First International Scientific Consensus Workshop; CRC: Boca Raton, F1, 1990. Austin, R. G. Degradation studies of polyolefins. In Degradable Materials: Perspectives, Issues and Opportunities; Barenberg, S. A., Brash, J. L., Narayan, R., Redpath, A. E., Eds.; The First International Scientific Consensus Workship; CRC: Boca Raton, FL, 1990. Benham, J. V.; Pullukat, T. J. Analysis of the types and amounts of carbonyl species present in oxidized polyethylene. J. Appl. Polym. Sci. 1976,20, 3295-3303. Dubois, M.;Gilles, K. A.; Hamilton, J. K.; Rebers, P. A.; Smith, F. Colorimetric method for determination of sugars and related substances. Anal. Chem. 1956,28, 350-356. Evangelista, R. L.;Sung, W.; Jane, J.; Gelina, R. J.; Nikolov, Z.L. Effect of compounding and starch modification on properties of starch-Tied low densitv Ind. Em. _wlvethvlene. .- - Chem. Res. 1991. 30,1841-1846. Fratzke. A. R.: Sune. -. W.: Evaneelista. R. L.: Nikolov. Z. L. Chemical method for'determination 2 starch in polyethylene. Anal. Lett. 1991,24,847-856. Gould, J. M.; Gordon, S. H.; Dexter, L. B.; Swanson, C. L. Biodegradation of starch-containing plastics. In Agriculatural and I
Ind. Eng. Chem. Res. 1992,31, 2339-2345 Synthetic Polymers; Glass, J. E., Swift, G., Eds.; ACS Syposium Series 433; American Chemical Society: Washington, DC, 1990; pp 65-75. Griffin, G. J. L. Biodegradable fillers in thermoplastic. Adv. Chem. Ser. 1974,134, 156-170. Griffin, G. J. L. Biodegradable synthetic resin sheet material containing starch and a fatty acid material. U.S. Patent 4016 117, 1977a. Griffin, G. J. L. Synthetic resin sheet material. U.S.Patent 4021 388, 1977b. Griffin, G. J. L. Degradable plastics. U.S.Patent 4983 651, 1991. Holmstrom, A,; Sorvik, E. M. Thermooxidative degradation of polyethylene. J. Polym. Sci. 1978, 16, 2555-2586. Iannotti, E.; Fair, N.; Tempesta, M.; Neibling, H.; Hsieh, F. H.; Mueller, R. Studies on the Environmental Degradation of Starch-Based Plastics. In Degradable Materials: Perspectives, Issues and Opportunities; Barenberg, S . A., Brash, J. L., Narayan, R., Redpath, A. E., Eds.; The First International Scientific Consensus Workship; CRC: Boca Raton, FL, 1990. Iring, M.; Tudos, F. Thermal oxidation of polyethylene and polypropylene. Prog. Polym. Sci. 1990, 15, 217-262. Jasse, B. In Food Packaging and Preservation; Mathlouthi, M., Ed.; Elsevier: London, 1986; Chapter 15. Lee, B.; Pometto, A. L.; Fratzke, A. R.; Bailey, T. B. Biodegradation of degradable plastic polyethylene by Phanerochaete and Streptomyces species. Appl. Environ. Microbiol. 1991, 57, 678-685. Maddever, W. J.; Campbell, P. D. Modified Starch Based Environmentally Degradable Plastics. In Degradable Materials: Perpectives, Issues and Opportunities; Barenberg, S . A,, Brash, J. L., Narayan, R., Redpath, A. E., Eds.; The First International Scientific Consensus Workshop; CRC: Boca Raton, FL, 1990.
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Nelson, N. A photometric adaptation of the Somogyi method for the determination of glucose. J. Biol. Chem. 1944, 153, 375-380. Otey, F. H.; Westhoff, R. P.; Russell, C. R. Biodegradable films from starch and ethylene-acrylic acid copolymer. Ind. Eng. Chem. Prod. Res. Dev. 1977, 16, 305-308. Otey, F. H.; Westhoff, R. P.; Doane, W. M. Starch-based blown films. Ind. Eng. Chem. Prod. Res. Dev. 1980,19,592-595. Otey, F. H.; Westhoff, R. P.; Doane, W. M. Starch-based blown films. 2. Znd. Eng. Chem. Res. 1987,26, 1659-1663. Peanasky, J. S.;Long, J. M.; Wool, R. P. Percolation effects in degradable polyethylene-starch blends. J. Polym. Sci., Polym. Phys. 1991,29, 565-579. Swanson, C. L.; Westhoff, R. P.; Doane, W. P. Modified starches in plastic films. In Proceedings of the Corn Utilization Conference ZI, Columbus, OH; National Corn Growers Association: St. Louis, MO, 1988. Swinkels, J. J. M. Sources of starch, its chemistry and physics. In Starch Conversion Technology; Van Beynum, G . M. A., Roels, J. A., Eds.; Marcel Dekker: New York, 1985. Wool, R. P.; Cole, M. A. Microbial degradation. In Engineering Materials Handbook; ASM International: Metals Park, OH, 1988; Vol. 2, pp 783-787. Wool, R. P.; Peanasky, J. M.; Long, J. M.; Goheen, S. M. Degradation mechanisms in polyethylene-starch blends. In Degradable Materials: Perspectives, Issues and Opportunities; Barenberg, S . A,, Brash, J. L., Naraym, R., Redpath, A. E., Eds.; The First International Scientific Consensus Workship; CRC Boca Raton, FL, 1990.
Received for review February 25, 1992 Accepted June 1, 1992
Drug Release Profiles in the Shrinking Process of Thermoresponsive Poly(N-isopropylacrylamide-co-alkyl methacrylate) Gels Ryo Yoshida and Kiyotaka Sakai Department of Chemical Engineering, Waseda University, 3-4-1, Ohkubo, Shinjuku-ku, Tokyo 169, Japan
Teruo Okano* and Yasuhisa Sakurai Institute of Biomedical Engineering, Tokyo Women’s Medical College, 8-1, Kawada-cho, Shinjuku-ku, Tokyo 162, Japan
Thermoresponsive poly (N-isopropylacrylamide-co-alkyl methacrylate) gels are capable of ”on-off“ regulation of drug release in response to external temperature changes because a gel surface skin formed with increasing temperature stops drug release from the gel interior. In this gel shrinking process, observation of bubble formation on the surface indicates that pressure is induced within the gel. This pressure may result in an outward convection of water. Drug must therefore be released not only by diffusion but also by convective transport. We have created a drug release model for this shrinking process using a tortuous pore model and simulated four decreasing patterns of release rate for different induction patterns of pressure. Experiments using indomethacin could match simulated release patterns by changing the chemical structure of polymer and thermal gradient. These changes induce different pressure fluctuations within gels and affect the release pattern from the gel “on” state to the “off“ state.
Introduction Recently, physiologically active therapeutic peptides have been artificially produced with genetic engineering techniques. These peptides are easily decomposed under physiological conditions and encounter absorption problems due to their high molecular weight. Hence, they cannot be utilized effectively in conventional dosage forms. Control of drug release rates in response to external stimuli would provide optimal therapeutic efficacy for such drugs having short pharmacologicql half-lives. Appropriate amounts of drug would be dosed via stimulus supplied exterior to the body only when drug is required. Such a system may lead to avoidance of drug tolerance by dosing
* To whom correspondence should be addressed.
in pulses, or achievement of an intelligent drug delivery system in which drug itself senses the signal caused from disease, judges the magnitude of signal, and then acta to release drug in direct response. To realize such modulated drug release system, polymer materials which change their structure and function in response to environmental change are attractive. Recently, temporal control of drug release has been attempted using stimuli-responsive polymers in response to specific chemical agents (Ishihara et al., 1984; Heller, 1988; Ito et al., 1989; Kitano et al., 1991), pH changes (Siegel, 1990,Dong and Hoffman, 1991), and electric fields (Sawahata et al., 1990; Kwon et al., 1990).
We have developed cross-linked poly(N-isopropylacrylamide) (PIPAAm)as a material for an intelligent drug delivery system responding to temperature. PIPAAm
0888-5885f 92f 2631-2339$03.00 f 0 0 1992 American Chemical Society
